1. Field of the Invention
This invention relates to a process for the polymerization of a conjugated diene, such as 1,3-butadiene, to a product having less than about 12, preferably less than 10% 1,2 microstructure. More specifically, it relates to the polymerization of the diene in hydrocarbon solution in intimate contact with a catalyst system comprising dihydrocarbyl magnesium and an alkali metal compound.
2. Related Prior Art
A number of Kamienski et al U.S. patents (U.S. Pat. Nos. 3,742,077, 3,822,219 and 3,847,883), disclose the use of a catalyst system comprising a combination of a dialkyl magnesium and alkyl lithium or alkyl potassium. These systems are disclosed as giving very low molecular weight polymers, namely in the range of about 2000 to 4000, even in the absence of telomerizing agents. U.S. Pat. No. 3,822,219 makes one reference (Example XVII) to the use of a complex (n-C.sub.4 H.sub.9 K).multidot.(n-C.sub.4 H.sub.9 MgO-tert.-C.sub.4 H.sub.9). Even though a telomerizing agent was used to give a molecular weight of 435, there is no indication that the molecular weight in this case would be any better than the other examples if the telomerizing agent was omitted. Moreover, in those examples where the 1,2 or vinyl content was reported, this was found to be 55-78%, which is undesirably high for many purposes.
U.S. Pat. No. 3,716,495 shows the polymerization of butadiene with a catalyst complex comprising an alkyl lithium with a polyalkyl compound such as diamylmagnesium (Ex. VII). However, the results are somewhat confused due to the fact that the invention is primarily directed to the use of modifying agents of polar type, such as ethers, thioethers and amines.
U.S. Pat. No. 3,817,955 discloses the polymerization of butadiene with a catalyst complex comprising a dialkyl magnesium with either an alkali metal hydride or a tetraalkylammonium hydride. However, here again a molecular weight of only 2100 is reported and the microstructure has 66.42% vinyl.
U.S. Pat. No. 3,846,385 discloses a polymerization catalyst system comprising dibutyl magnesium and a barium dialkoxide such as barium di-tertiarybutoxide. However, this patent gives 80-85% trans-1,4 instead of the more desirable cis-1,4.
U.S. Pat. No. 3,324,099 shows a polymerization initiator comprising a complex formed from an alkyl lithium and a bis-benzylmercaptan compound which is modified in some cases with a tetrahydrocarbyl tin compound.
These cited prior art patents show a variety of catalyst systems giving a variety of results, some favorable and some unfavorable.